The present invention is generally directed toward vessels for storing hydrogen fuels onboard vehicles.
Petroleum fuels are currently the primary fuels for operating internal combustion engines and turbines in vehicles, generators and many other applications. Gasoline and diesel, for example, are currently the most popular fuels for operating cars, trucks, machinery and other motorized equipment. It is estimated that the transportation sector consumes nearly 50% of the total petroleum fuels consumed in the United States. One problem of using petroleum fuels is that they produce a significant amount of air pollution. The United States recognized this problem in the 1990 Clean Air Act and the 1992 Energy Policy Act. Another serious problem of using petroleum fuels is that the United States and other industrialized countries import more than 50% of the oil that they consume. As a result, the economies and the national security of many industrialized countries are susceptible to production controls and foreign policy concerns of foreign petroleum producing countries. Therefore, it is well recognized that there is a high demand for systems that can generate, distribute and use abundant and clean transportation fuels.
Hydrogen is one of the most promising fuels that is being considered to replace petroleum fuels for the transportation sector. In the case of vehicles, hydrogen fuel-cells that generate electricity from a flow of hydrogen are being used to power electric automobile engines, and combustion engines that burn hydrogen are being used in other applications. One advantage of using hydrogen is that it does do not produce air pollution. An advantage of using hydrogen fuel-cells is that vehicles will not need to carry large, heavy batteries to store electrical power because the hydrogen fuel-cells provide a power plant onboard the vehicles. As a result, electrical vehicles with hydrogen fuel-cells are expected to be lighter and more efficient than existing battery-powered electrical vehicles. Hydrogen fuels also provide more energy than either gasoline or natural gas on a per-weight basis, and hydrogen is also readily abundant from resources within the borders of the United States and other industrialized countries. Hydrogen fuels may accordingly reduce the economic and foreign policy concerns caused by importing a significant percentage of the petroleum fuels. Therefore, it would be very beneficial to replace gasoline and diesel with hydrogen as a viable fuel for the transportation sector.
The implementation of a national energy economy based on hydrogen fuels will require the development of many systems and processes to make hydrogen fuels as safe and convenient to use as gasoline or diesel. One area of hydrogen fuel technology that needs further development is storing hydrogen onboard a vehicle. Although hydrogen has more energy than gasoline on a per-weight basis, it has a much lower energy/volume than gasoline. As a result, conventional hydrogen storage systems require a much larger storage vessel than gasoline tanks to provide the same operating range for a vehicle. In most vehicles, however, the space allotted for storing fuels is much smaller than the volume required for an onboard hydrogen storage vessel. Therefore, a significant amount of research and development is being directed toward providing cost-effective storage vessels that can store a sufficient amount of hydrogen within the limited volume of an onboard fuel tank to provide approximately the same range as conventional gasoline powered vehicles.
Existing systems for storing hydrogen onboard vehicles include containers of compressed or liquefied hydrogen, and hydrogen stored in metal hydrides. According to the Department of Energy, the energy density goals for storing hydrogen onboard vehicles are 6.5 weight percent H2 and 62 kg H2/m3. Existing storage systems for compressed or liquefied hydrogen are generally high-pressure storage vessels with a vacant cavity that can hold approximately 6.7 weight percent H2 and 20 kg H2/m3 at a pressure of 5000 psi. Although it is possible to increase the energy density of hydrogen in high-pressure storage vessels by increasing the pressure, it not only takes a significant amount of energy to pressurize the gas in such vessels, but the storage vessels must also be more robust to withstand the higher pressures. As a result, it may not be feasible to achieve an adequate energy density to match the operating range of conventional gasoline powered vehicles with existing high-pressure storage vessels.
Another system for storing hydrogen, which is less developed than high-pressure storage vessels, is gas-on-solid adsorption. A particularly promising gas-on-solid adsorption material is a carbon nanotube structure, which can have single-wall carbon nanotubes and multi-wall carbon nanotubes. Single-wall carbon nanotubes are single elongated cylinders of carbon, and multi-wall carbon nanotubes have concentrically arranged cylinders of elongated carbon (i.e., a tube within a tube). The diameter of the carbon cylinders is determined by the manufacturing process and can be less than 2 nm, and the nanotubes can be formed into bundles of generally parallel nanotubes because of van der Waals interaction. In general, a bundle of carbon nanotubes form a porous medium in which H2 atoms fill the pores by capillary action. In theory, it has been shown that mediums formed from carbon nanotubes can store up to 8.4 weight percent H2 and 82 kg H2/m3. Although such an energy density is highly desirable, it may be difficult to achieve this energy density outside of laboratory conditions. Therefore, carbon nanotubes alone may not provide an adequate energy density to meet the goals set by the Department of Energy for commercial transportation applications.
The state of the art of existing and experimental systems for storing hydrogen onboard vehicles currently falls short of providing the same range between fill-ups as conventional gasoline powered vehicles. Thus, it would be highly desirable to develop an onboard hydrogen storage system that can (a) fit within the limited amount of space provided for fuel tanks on most vehicles and (b) store enough hydrogen to power a vehicle for a range of approximately 300 miles.
The present invention is directed toward devices and methods for storing hydrogen fuels or other gaseous fuels. One embodiment of a container for storing gaseous fuels in accordance with an aspect of the invention comprises a high-pressure vessel configured to contain the gas in a high-pressure zone at a pressure significantly above atmospheric pressure. The container can also include a storing medium in the vessel and an inlet/outlet line extending through the vessel. The storing medium can have a plurality of storage spaces configured to physically bind molecules of the gas to the storing medium, and the inlet/outlet line can be a tube extending through the vessel in fluid communication with the storing medium. In operation, the high-pressure vessel is pressurized with a gaseous fuel (e.g., hydrogen) to a pressure significantly above atmospheric pressure (e.g., approximately 3,000-10,000 psi). The molecules of the gaseous fuel bind to the storing medium, and the pressure in the vessel drives additional molecules of the gaseous fuel into vacant spaces within the storing medium or in other regions of the vessel.
In one embodiment, the high-pressure vessel comprises a composite shell having a closed-end and an open-end, an end-cover attached to the open-end of the shell to define a high-pressure zone, and a divider configured to separate the high-pressure zone into cells. The storing medium can be an adsorbent material comprising carbon nanotubes configured to adsorb H2 molecules, and the storing medium can be positioned in the cells. Additionally, the inlet/outlet line can be a porous tube extending along virtually the full length of the high-pressure zone, and the vessel can further include an in-tank regulator or external regulator coupled to the inlet/outlet line.